feat: instance-level SSH key selection for container mounting

- Revert mistaken ssh_key_id from ConfigProfile (model, API, resolver, frontend)
- Add ssh_key_ids JSON column to tool_instances via migration
- Update create_instance to accept and store ssh_key_ids
- Update start_instance to mount selected SSH keys to {home_dir}/.ssh
- Update list_instances to return ssh_key_ids
- Frontend CreateSessionForm: multi-select SSH key checkboxes
- Frontend instance-list: SSH key selector for start/restart actions
- Maintain separate SSH key dirs per key to avoid conflicts

Quality gates: pytest (231 passed, 6 pre-existing), tsc --noEmit clean

apps/api/src/models/config_profile.py

@@ -9,7 +9,6 @@ from src.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
 
 if TYPE_CHECKING:
     from src.models.project import Project
-    from src.models.ssh_key import SSHKey
     from src.models.tool_type import ToolType
     from src.models.user import User
 
@@ -43,15 +42,11 @@ class ConfigProfile(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     git_mounts: Mapped[list] = mapped_column(
         JSON, default=list, nullable=False
     )  # [{"remote_url": "https://github.com/user/repo.git", "source_path": ".", "target_path": "/path", "branch": "main"}, ...]
-    ssh_key_id: Mapped[uuid.UUID | None] = mapped_column(
-        UUID(), ForeignKey("ssh_keys.id", ondelete="SET NULL"), nullable=True
-    )
     is_default: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
 
     user: Mapped["User"] = relationship()
     project: Mapped["Project | None"] = relationship()
     tool_type: Mapped["ToolType | None"] = relationship()
-    ssh_key: Mapped["SSHKey | None"] = relationship()
     includes: Mapped[list["ConfigProfileInclude"]] = relationship(
         "ConfigProfileInclude",
         foreign_keys="ConfigProfileInclude.profile_id",

apps/api/src/models/tool_instance.py

@@ -59,6 +59,7 @@ class ToolInstance(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     selected_config_profile_id: Mapped[uuid.UUID | None] = mapped_column(
         UUID(), ForeignKey("config_profiles.id", ondelete="SET NULL"), nullable=True
     )
+    ssh_key_ids: Mapped[list[str] | None] = mapped_column(JSON, nullable=True)
 
     tool_type: Mapped["ToolType"] = relationship()
     repository: Mapped["GitRepository"] = relationship()